According to *Star Trek: Final Frontier* by Diane Carey, the *USS Constitution* had been redesigned so many times in construction that its first captain, Robert April, and first officer George Kirk (James Kirk's father) re-christened the ship *Enterprise.* [enter link description here][1]

Diane Carey wrote a Star Trek novel called "Final Frontier." In it, Captain Robert April and First Officer George Kirk flew the maiden voyage of the USS Enterprise, which was originally called the USS Constitution; Pike wanted to rename the ship because it was far more advanced than the Constitution that was on the blueprints, and Kirk came up with the name "Enterprise." All clear now?

The question didn't specify whether it was referring to drafts of the pilot script or in the series' timeline, so both answers are correct; in early scripts, it was the Yorktown, in the novel it was the Constitution - and plenty of places state that the Enterprise is a Constitution-class starship, and the class is usually named for the first ship to be built. Several places state that the actual USS Constitution was built but never commissioned (operated as a Starfleet vessel), which agrees with Diane Carey's novel.

At the risk of pointing out the obvious, it seems like Gene Robbenberry, as the creator of the series, might have had more of a handle on what the ship was originally going to be named than an author who began writing Star Trek novels 20 or so years after the fact... and who had no part in the production of the original series.

Also, I seem to recall author David Gerrold mentioning in his book about the production of the original series (sorry, can't remember its name) that there were actually memos that discussed the ship's name change, from Yorktown to Enterprise.

Gene RODDENBERRY created the series, and while the ship was the USS Yorktown in unused scripts, that is not canon; there is no movie or series episode that has ever mentioned it was once the "Yorktown."